This application relates to power supply circuits in general, and in particular, to a power supply circuit utilizing a crowbar. A typical prior art power supply circuit utilizing a crowbar is illustrated in FIG. 1.
In this conventional circuit, a power supply provides power to a load and a circuit interruption element, such as fuse 11, is utilized to disconnect the load from the power supply in the event of an over-current condition. A crowbar circuit is connected to the power supply output. The crowbar circuit includes over-voltage sensing means. In the event of a failure in the power supply resulting in an excessive voltage at its output, the crowbar circuit is actuated to short the power supply output to ground.
By shorting to ground, substantial current is drawn through the fuse 11 causing it to open and thereby disconnect the load from the power supply. This crowbar circuit is utilized to prevent possible damage to the load due to the overvoltage condition.
A power source utilizing a DC to DC converter for powering a microprocessor is illustrated in FIG. 2. A full disclosure of such a system is given in U.S. Pat. No. 4,355,277 issued Oct. 19, 1982, and entitled Dual-Mode DC/DC Converter, the disclosure of which is hereby incorporated by reference.
When it is desired to use a single cell battery or other low voltage power source, a DC to DC converter can be used to provide an appropriate stepped up output voltage such as 3 volts, to power a microprocessor circuit or other load. An output capacitor 13 is provided for filtering the output waveform of the DC to DC converter. A switch (not shown) can be utilized to connect the power source to the DC to DC converter.
In certain modes of operation, a microprocessor can represent a very small load on a circuit. For example, a CMOS microprocessor which is only being utilized for counting or timing purposes may draw as little as 5 to 10 microamps of current at 3 volts. This small current drawn by the microprocessor can present a problem in that even when the DC to DC converter is turned off, the filter capacitor 13 can supply the actual voltage and current requirements of the microprocessor for a finite but considerable time interval.
The fact that the microprocessor can operate from this stored charge becomes a significant problem in applications such as radio pagers in which the microprocessor is used as a controller. In the case of paging receivers and many similar devices, the devices are commonly designed to generate an audible or visual turn-on signal when the power switch is turned on to indicate that the device is operating properly, and in the case of battery powered equipment, that the batteries contain sufficient energy to operate the device.
In applications of this type, it is important that the control circuitry be reset or initialized quickly whenever the power switch is turned off so that a turn on alert signal can be generated as soon as the power switch is turned on. In the specific case of the microprocessor controller used in a paging receiver, it thus becomes desirable to reset the microprocessor every time the power switch is turned OFF.
Previous to the use of microprocessors as decoders or controllers in paging receivers, conventional analog and digital circuits that operated directly from the battery voltage were used as decoders, and these circuits were quickly reset by the sudden drop in supply voltage when the power source was disconnected. The combined use of a DC-DC converter and a microprocessor presents a new problem in that momentary interruptions in the power source do not effect the output voltage of the DC-DC converter due to the stored charge on the filter capacitor, with the result that fairly long disruptions in the power source do not reset the microcomputer. The net effect is that a pager user might turn the device off, then turn it on again several seconds later and the pager may not generate a turn on alert signal leading the user to believe that his pager battery is dead.
This invention provides a solution to this problem.